A study of the variable factors controlling spray drying

Abstract

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The process of spray drying consists of dispersing a liquid to be dried into a hot desiccating medium. This operation is carried out in a chamber where the process is completed, and from which the dried product is removed. Due to the tremendous surface area of the dispersed particles and intimate contact of the particles with the drying medium, the drying time is reduced to a minimum These facts along with the fact that the particle is not heated above the wet bulb temperature of the drying medium, make the process particularly adaptable to drying heat sensitive materials.

Although, the theory for spray drying is similar to that for other drying processes, it is complicated by the aerodynamics of small particles suspended in a turbulent medium. Neither quantitative data nor mathematical relationships are available in the literature which would enable spray drying equipment to be designed on the basis of calculated drying rates. A study of the numerous variables encountered in drying materials by this process should be very helpful for future design work.

The purpose of this investigation was to determine how variable factors such as condition of feed, atomizing variables, and chamber conditions affected the moisture content, bulk density, and particle size of a spray dried material. These variables were studied in a unit constructed at the Virginia Polytechnic Institute according to plans based on available information, and cut and try experimental work.

Sodium sulphate was dried in this unit with the concentration, temperature and pressure of the feed; temperature and pressure of the atomizing air; and temperature of the drying air varied from an arbitrary set of operating conditions. The product from these runs was analyzed for its moisture content, bulk density, and particle size. The results of these analysis indicated that the concentration of the feed was the most significant variable tested. A variation of 25 to 200 per cent saturation at room temperature reduced the specific surface from 24.04 to 20.00, reduced the bulk density from 2.5 to 2.2 grams per cc and also reduced the moisture content from 1.16 to 0.55 per cent. Increasing the temperature of the atomizing air from 75 to 200° F and temperature of the atomizing air from 95 to 200° F was found to have almost no effect on the variables tested. Varying the atomizing air pressure from 8 to 26 pounds per sq. in. reduced the moisture content from 0.58 to 0.47 per cent and increased the bulk density of the product from 2.2 to 2.4 grams per cc without any definite trend in specific surface. The bulk density was decreased from 2.5 to 2.5 grams per cc when the temperature of the chamber air was increased from 150 to 236° F. Variation in the specific surface factor indicated that the particle size increased and the moisture content remained constant for this run.